Tunable acoustic gradient lens system utilizing amplitude adjustments for acquiring images focused at different z-heights

ABSTRACT

A variable focal length lens system includes a tunable acoustic gradient (TAG) lens; a TAG lens controller; and processor(s) configured to: (a) operate the TAG lens controller to drive a periodic modulation of the TAG lens optical power at a TAG lens resonant frequency, using a first amplitude driving signal that provides a first focal Z range extending between peak focus distances Z1max+ and Z1max−; and expose a first image using a first exposure increment approximately at the timing of Z1max+ or Z1max−; (b) adjust the TAG lens controller to drive the periodic modulation using a second amplitude driving signal that provides a second focal Z range extending between peak focus distances Z2max+ and Z2max−; and expose a second image using a second exposure increment approximately at the timing of Z2max+ or Z2max−; and (c) perform processing that includes determining focus metric values for the first and second images.

FIELD

This disclosure relates to precision metrology, and more particularly tomachine vision inspection systems and other optical systems in which avariable focal length lens such as a tunable acoustic gradient (TAG)lens may be utilized to periodically modulate a focus position.

BACKGROUND

Precision non-contact metrology systems such as precision machine visioninspection systems (or “vision systems” for short) may be utilized toobtain precise dimensional measurements of objects and to inspectvarious other object characteristics, and may include a computer, acamera and optical system, and a precision stage that moves to allowworkpiece traversal and inspection. One exemplary prior art system isthe QUICK VISION® series of PC-based vision systems and QVPAK® softwareavailable from Mitutoyo America Corporation (MAC), located in Aurora,Ill. The features and operation of the QUICK VISION® series of visionsystems and the QVPAK® software are generally described, for example, inthe QVPAK 3D CNC Vision Measuring Machine User's Guide, publishedJanuary 2003, which is hereby incorporated herein by reference in itsentirety. This type of system uses a microscope-type optical system andmoves the stage to provide inspection images of either small orrelatively large workpieces.

General-purpose precision machine vision inspection systems aregenerally programmable to provide automated video inspection. Suchsystems typically include selectable modes of operation as well as GUIfeatures and predefined image analysis “video tools,” such thatoperation and programming can be performed by “non-expert” operators.For example, U.S. Pat. No. 6,542,180, which is hereby incorporatedherein by reference in its entirety, teaches a vision system that usesautomated video inspection including the use of various video tools.

Multi-lens variable focal length (VFL) optical systems may be utilizedfor observation and precision measurement of surface heights, and may beincluded in a microscope and/or precision machine vision inspectionsystem, for example as disclosed in U.S. Pat. No. 9,143,674, which ishereby incorporated herein by reference in its entirety. Briefly, a VFLlens is capable of acquiring multiple images at multiple focal lengths,respectively. One type of known VFL lens is a tunable acoustic gradient(“TAG”) lens that creates a lensing effect using sound waves in a fluidmedium. The sound waves may be created by application of an electricalfield at a resonant frequency to a piezoelectric tube surrounding thefluid medium to create a time-varying density and index of refractionprofile in the lens's fluid, which modulates its optical power andthereby the focal length (or effective focus position) of the opticalsystem. A TAG lens may be used to periodically sweep a range of focallengths (i.e., to periodically modulate its optical power) at a resonantfrequency greater than 30 kHz, or greater than 70 kHz, or greater than100 kHz, or greater than 400 kHz, up to 1.0 MHz for example, at a highspeed. Such a lens may be understood in greater detail by the teachingsof the article, “High speed varifocal imaging with a tunable acousticgradient index of refraction lens” (Optics Letters, Vol. 33, No. 18,Sep. 15, 2008), which is hereby incorporated herein by reference in itsentirety. TAG lenses and related controllable signal generators areavailable, for example, from Mitutoyo Corporation of Kanagawa, Japan. Asa specific example, SR38 series TAG lenses are capable of periodicmodulation having a modulation frequency of up to 1.0 MHz. Variousaspects of operating principles and applications of TAG lenses aredescribed in greater detail in U.S. Pat. Nos. 9,930,243; 9,736,355;9,726,876; 9,143,674; 8,194,307; and 7,627,162; and in US PatentApplication Publication Nos. 2017/0078549 and 2018/0143419, each ofwhich is hereby incorporated herein by reference in its entirety.

While such imaging systems including a TAG lens can change the effectivefocus position at a very high rate, the amount of lighting that can beprovided during an image exposure for a given amount of focus change maybe relatively limited. An imaging system that can provide improvementswith regard to such issues would be desirable.

BRIEF SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

A variable focal length (VFL) lens system is provided including atunable acoustic gradient (TAG) lens, a TAG lens controller, a lightsource, and objective lens, a camera, a memory and one or moreprocessors. The TAG lens controller controls the TAG lens toperiodically modulate the optical power of the TAG lens over a range ofoptical powers at an operating frequency. The objective lens inputsworkpiece light arising from a first workpiece surface, which isilluminated by the light source, and transmits the workpiece light alongan imaging optical path that passes through the TAG lens. The camerareceives the workpiece light transmitted by the TAG lens along theimaging optical path and provides a corresponding workpiece imageexposure.

The memory stores programmed instructions and the one or more processorsto execute the programmed instructions to perform operations such as thefollowing. During a first timeframe: the TAG lens controller is operatedto drive a periodic modulation of the TAG lens optical power at aresonant frequency of the TAG lens, using a first amplitude drivingsignal that provides a first amplitude of the periodic modulation at theresonant frequency, the first amplitude corresponding to a first focal Zrange extending between peak focus distances Z1max+ and Z1 max−; and thelight source and camera are operated to expose a first image using afirst exposure increment having a first exposure increment time intervalthat is approximately centered at the timing of either the peak focusdistance Z1max+ or Z1 max−. During a second timeframe: the TAG lenscontroller is adjusted to drive the periodic modulation of the TAG lensoptical power at the resonant frequency of the TAG lens, using a secondamplitude driving signal that provides a second amplitude of theperiodic modulation at the resonant frequency that is different than thefirst amplitude of the periodic modulation, the second amplitudecorresponding to a second focal Z range extending between peak focusdistances Z2max+ and Z2max−, wherein the second focal Z range isdifferent than the first focal Z range; and the light source and cameraare operated to expose a second image using a second exposure incrementhaving a second exposure increment time interval that is approximatelycentered at the timing of either the peak focus distance Z2max+ orZ2max−. In addition, processing is performed that includes determiningfocus metric values for the first and second images.

In various implementations, the processing includes comparing the focusmetric values for the first and second images. In variousimplementations, the processing includes determining that a focus metricvalue for the second image is higher than a focus metric value for thefirst image. In various implementations, the first image and the secondimage are part of an image stack obtained by the system operating in apoints from focus (PFF) mode, and the focus metric values are processedto determine a Z-height of the first workpiece surface.

In various implementations, the focus distance during the first exposureincrement moves over a first exposure Z range that is less than 10% ofthe first focal Z range. In various implementations, the focus distanceduring the first exposure increment moves over a first exposure Z rangethat is less than 2 DOF (depth of field) of the system. In variousimplementations, the focus distance during the second exposure incrementmoves over a second exposure Z range that is less than 10% of the firstfocal Z range and/or less than 10% of the second focal Z range.

In various implementations, the first exposure increment time intervalis equal to the second exposure increment time interval. In variousalternative implementations, the first exposure increment time intervalis different than the second exposure increment time interval.

In various implementations, the one or more processors are configured toexecute the programmed instructions to further perform operations suchas the following. During a third timeframe: the TAG lens controller isadjusted to drive the periodic modulation of the TAG lens optical powerat the resonant frequency of the TAG lens, using a third amplitudedriving signal that provides a third amplitude of the periodicmodulation at the resonant frequency that is different than the firstamplitude and the second amplitude of the periodic modulation, the thirdamplitude corresponding to a third focal Z range extending between peakfocus distances Z3max+ and Z3max−, wherein the third focal Z range isdifferent than the first focal Z range and the second focal Z range; andthe light source and camera are operated to expose a third image using athird exposure increment having a third exposure increment time intervalthat is approximately centered at the timing of either the peak focusdistance Z3max+ or Z3max−. In addition, the processing that is performedmay include determining focus metric values for the first, second andthird images.

A method is provided for operating a variable focal length (VFL) lenssystem comprising a tunable acoustic gradient (TAG) lens and with aworkpiece surface in a measurement/imaging volume of the VFL lenssystem. The method includes the following steps. The optical power ofthe TAG lens is periodically modulated over a range of optical powers ata resonant frequency of the TAG lens. During a first timeframe: theperiodic modulation of the TAG lens optical power is driven, using afirst amplitude driving signal that provides a first amplitude of theperiodic modulation at the resonant frequency, the first amplitudecorresponding to a first focal Z range extending between peak focusdistances Z1max+ and Z1 max−; and a light source and a camera areoperated to expose a first image using a first exposure increment havinga first exposure increment time interval that is approximately centeredat the timing of either the peak focus distance Z1max+ or Z1 max−.During a second timeframe: a periodic modulation of the TAG lens opticalpower is driven, using a second amplitude driving signal that provides asecond amplitude of the periodic modulation at the resonant frequencythat is different than the first amplitude of the periodic modulation,the second amplitude corresponding to a second focal Z range extendingbetween peak focus distances Z2max+ and Z2max−, wherein the second focalZ range is different than the first focal Z range; and the light sourceand camera are operated to expose a second image using a second exposureincrement having a second exposure increment time interval that isapproximately centered at the timing of either the peak focus distanceZ2max+ or Z2max−. In addition, processing is performed that includesdetermining focus metric values for the first and second images.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a diagram showing various typical components of ageneral-purpose precision machine vision inspection system;

FIG. 2 is a block diagram of a control system portion and a visioncomponents portion of a machine vision inspection system similar to thatof FIG. 1 and including certain features disclosed herein;

FIG. 3 is a schematic diagram of a VFL (TAG) lens system that may beadapted to a precision non-contact metrology system such as a machinevision inspection system, wherein the TAG lens system is capable ofoperating in a standard mode (phase mode) and an amplitude adjustmentmode of controlling a lens focus position;

FIG. 4A is a timing diagram illustrating phase timings for aperiodically modulated control signal and optical response of the VFL(TAG) lens system of FIG. 3, which is operating in the standard (phase)mode, and also qualitatively showing how strobed illumination can betimed to correspond with a respective phase timing of the periodicallymodulated focus position to expose an image focused at a respective Zheight;

FIG. 4B is another timing diagram illustrating phase timings for aperiodically modulated control signal and optical response (aperiodically modulated Z height of the focus position) of the VFL (TAG)lens system of FIG. 3, which is operating in the standard (phase) mode;

FIG. 5 is a timing diagram illustrating a first amplitude of theperiodic modulation of the VFL (TAG) lens system of FIG. 3 in responseto a first amplitude driving signal as operating in the amplitudeadjustment mode, and also illustrating for comparison certain phasetimings as would be utilized in a standard phase mode;

FIGS. 6A-6F respectively illustrate different amplitudes of periodicmodulation of the VFL (TAG) lens system of FIG. 3 as resulting fromdifferent amplitude driving signals;

FIG. 7 is a flow diagram showing one example of a method for operating aVFL (TAG) lens system with amplitude adjustment corresponding toZ-height according to principles disclosed herein; and

FIG. 8 is a flow diagram showing one example of a method for operating aVFL (TAG) lens system utilizing amplitude adjustments for acquiringimages focused at different Z-heights according to principles disclosedherein.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of one exemplary machine vision inspectionsystem 10 usable as or including a VFL (TAG) lens system (alsoreferenced herein as an imaging system) in accordance with methodsdescribed herein. The machine vision inspection system 10 includes avision measuring machine 12 that is operably connected to exchange dataand control signals with a controlling computer system 14. Thecontrolling computer system 14 is further operably connected to exchangedata and control signals with a monitor or display 16, a printer 18, ajoystick 22, a keyboard 24, and a mouse 26. The monitor or display 16may display a user interface suitable for controlling and/or programmingthe operations of the machine vision inspection system 10. It will beappreciated that, in various implementations, a touchscreen tablet orthe like may be substituted for and/or redundantly provide the functionsof any or all of the elements 14, 16, 22, 24 and 26.

Those skilled in the art will appreciate that the controlling computersystem 14 may generally be implemented using any suitable computingsystem or device, including distributed or networked computingenvironments, and the like. Such computing systems or devices mayinclude one or more general-purpose or special-purpose processors (e.g.,non-custom or custom devices) that execute software to perform thefunctions described herein. Software may be stored in memory, such asrandom-access memory (RAM), read-only memory (ROM), flash memory, or thelike, or a combination of such components. Software may also be storedin one or more storage devices, such as optical-based disks, flashmemory devices, or any other type of non-volatile storage medium forstoring data. Software may include one or more program modules thatinclude routines, programs, objects, components, data structures, and soon that perform particular tasks or implement particular abstract datatypes. In distributed computing environments, the functionality of theprogram modules may be combined or distributed across multiple computingsystems or devices and accessed via service calls, either in a wired orwireless configuration.

The vision measuring machine 12 includes a moveable workpiece stage 32and an optical imaging system 34 that may include a zoom lens orinterchangeable objective lenses. The zoom lens or interchangeableobjective lenses generally provide various magnifications for the imagesprovided by the optical imaging system 34. Various implementations of amachine vision inspection system 10 are also described in commonlyassigned U.S. Pat. Nos. 7,454,053; 7,324,682; 8,111,905; and 8,111,938,each of which is hereby incorporated herein by reference in itsentirety.

FIG. 2 is a block diagram of a control system portion 120 and a visioncomponents portion 200 of a machine vision inspection system 100 similarto the machine vision inspection system of FIG. 1, including certainfeatures disclosed herein. As will be described in more detail below,the control system portion 120 is utilized to control the visioncomponents portion 200. The vision components portion 200 includes anoptical assembly portion 205, light sources 220, 230, 240, and aworkpiece stage 210 having a central transparent portion 212. Theworkpiece stage 210 is controllably movable along X and Y axes that liein a plane that is generally parallel to the surface of the stage wherea workpiece 20 may be positioned.

The optical assembly portion 205 includes a camera system 260, aninterchangeable objective lens 250 and a variable focal length (VFL)lens 270 (e.g., a TAG lens in various exemplary implementations). Invarious implementations, the optical assembly portion 205 may furtherinclude a turret lens assembly 223 having lenses 226 and 228. As analternative to the turret lens assembly, in various implementations, afixed or manually interchangeable magnification-altering lens, or a zoomlens configuration, or the like, may be included. In variousimplementations, the interchangeable objective lens 250 may be selectedfrom a set of fixed magnification objective lenses that are included aspart of the variable magnification lens portion (e.g., a set ofobjective lenses corresponding to magnifications such as 0.5×, 1×, 2× or2.5×, 5×, 7.5×, 10×, 20× or 25×, 50×, 100×, etc.).

The optical assembly portion 205 is controllably movable along a Z axisthat is generally orthogonal to the X and Y axes by using a controllablemotor 294 that drives an actuator to move the optical assembly portion205 along the Z axis to change the focus of the image of a workpiece 20.The controllable motor 294 is connected to an input/output interface 130via a signal line 296. As will be described in more detail below, tochange the focus of the image over a smaller range, or as an alternativeto moving the optical assembly portion 205, the VFL (TAG) lens 270 maybe controlled via a signal line 234′ by a lens control interface 134 toperiodically modulate the optical power of the VFL lens 270 and thusmodulate an effective focus position of the optical assembly portion205. The lens control interface 134 may include a VFL lens controller180 according to various principles disclosed herein, as described ingreater detail below. A workpiece 20 may be placed on the workpiecestage 210. The workpiece stage 210 may be controlled to move relative tothe optical assembly portion 205, such that the field of view of theinterchangeable objective lens 250 moves between locations on aworkpiece 20, and/or among a plurality of workpieces 20, etc.

One or more of a stage light source 220, a coaxial light source 230, anda surface light source 240 (e.g., a ring light) may emit source light222, 232, and/or 242, respectively, to illuminate a workpiece 20 orworkpieces 20. In various exemplary embodiments, strobed illuminationmay be used. For example, during an image exposure, the coaxial lightsource 230 may emit strobed source light 232 along a path including abeam splitter 290 (e.g., a partial mirror). The source light 232 isreflected or transmitted as image light 255, and the image light usedfor imaging passes through the interchangeable objective lens 250, theturret lens assembly 223 and the VFL (TAG) lens 270, and is gathered bythe camera system 260. A workpiece image exposure, which includes theimage of the workpiece(s) 20, is captured by the camera system 260, andis output on a signal line 262 to the control system portion 120.

Various light sources (e.g., the light sources 220, 230, 240) may beconnected to a lighting control interface 133 of the control systemportion 120 through associated signal lines (e.g., busses 221, 231, 241,respectively). The control system portion 120 may control the turretlens assembly 223 to rotate along axis 224 to select a turret lensthrough a signal line or bus 223′ to alter an image magnification.

As shown in FIG. 2, in various exemplary implementations, the controlsystem portion 120 includes a controller 125, the input/output interface130, a memory 140, a workpiece program generator and executor 170, and apower supply portion 190. Each of these components, as well as theadditional components described below, may be interconnected by one ormore data/control busses and/or application programming interfaces, orby direct connections between the various elements. The input/outputinterface 130 includes an imaging control interface 131, a motioncontrol interface 132, a lighting control interface 133, and the lenscontrol interface 134. The lens control interface 134 may include or beconnected to a VFL lens controller 180 including circuits and/orroutines for controlling various image exposures synchronized with theperiodic focus position modulation provided by the VFL (TAG) lens 270.In some implementations, the lens control interface 134 and the VFL lenscontroller 180 may be merged and/or indistinguishable.

The lighting control interface 133 may include lighting control elements133 a-133 n that control, for example, the selection, power, on/offswitch, and strobe pulse timing, if applicable, for the variouscorresponding light sources of the machine vision inspection system 100.In some implementations, an exposure (strobe) time controller 333 es asshown in FIG. 3 may provide strobe timing signals to one or more of thelighting control elements 133 a-133 n, such that they provide an imageexposure strobe timing that is synchronized with a desired phase time ofthe VFL lens focus position modulation (e.g., in accordance with certainstored calibration data), and as described in greater detail below. Insome implementations, the exposure (strobe) time controller 333 es andone or more of the lighting control elements 133 a-133 n may be mergedand/or indistinguishable.

The memory 140 may include an image file memory portion 141, anedge-detection memory portion 140 ed, a standard mode (phase mode)portion 140 ph, an amplitude adjustment mode portion 140 am, a workpieceprogram memory portion 142 that may include one or more part programs,or the like, and a video tool portion 143. The standard mode (phasemode) portion 140 ph is responsible for controlling a lens focusposition of the TAG lens 270 using phase timings of a periodicallymodulated control signal for the TAG lens 270, as described more fullybelow in reference to FIGS. 4A and 4B. The amplitude adjustment modeportion 140 am is responsible for controlling a lens focus position ofthe TAG lens 270 using amplitude driving signals, as described morefully below in reference to FIGS. 5 and 6A-6F.

The video tool portion 143 includes video tool portion 143 a and othervideo tool portions (e.g., 143 n) that determine the GUI,image-processing operation, etc., for each of the corresponding videotools, and a region of interest (ROI) generator 143 roi that supportsautomatic, semi-automatic, and/or manual operations that define variousROIs that are operable in various video tools included in the video toolportion 143. Examples of the operations of such video tools for locatingedge features and performing other workpiece feature inspectionoperations are described in more detail in certain of the previouslyincorporated references, as well as in U.S. Pat. No. 7,627,162, which ishereby incorporated herein by reference in its entirety.

The video tool portion 143 includes an autofocus video tool 143 af thatdetermines the GUI, image-processing operation, etc., for focus height(i.e., effective focus position (Z-height)) measurement operations. Invarious implementations, the autofocus video tool 143 af mayadditionally include a high-speed focus height tool that may be utilizedto measure focus heights with high speed using hardware illustrated inFIG. 3, as described in more detail in U.S. Pat. No. 9,143,674, which isincorporated above. In various implementations, the high-speed focusheight tool may be a special mode of the autofocus video tool 143 afthat may otherwise operate according to conventional methods forautofocus video tools, or the operations of the autofocus video tool 143af may only include those of the high-speed focus height tool.High-speed autofocus and/or focus position determination for an imageregion or regions of interest may be based on analyzing the image todetermine a corresponding focus characteristic value (e.g., aquantitative contrast metric value and/or a quantitative focus metricvalue) for various regions, according to known methods. For example,such methods are disclosed in U.S. Pat. Nos. 8,111,905; 7,570,795; and7,030,351, each of which is hereby incorporated herein by reference inits entirety.

In the context of this disclosure, and as is known by one of ordinaryskill in the art, the term “video tool” generally refers to a relativelycomplex set of automatic or programmed operations that a machine visionuser can implement through a relatively simple user interface. Forexample, a video tool may include a complex pre-programmed set ofimage-processing operations and computations that are applied andcustomized in a particular instance by adjusting a few variables orparameters that govern the operations and computations. In addition tothe underlying operations and computations, the video tool comprises theuser interface that allows the user to adjust those parameters for aparticular instance of the video tool. It should be noted that thevisible user interface features are sometimes referred to as the videotool, with the underlying operations being included implicitly.

One or more display devices 136 (e.g., the display 16 of FIG. 1) and oneor more input devices 138 (e.g., the joystick 22, keyboard 24, and mouse26 of FIG. 1) may be connected to the input/output interface 130. Thedisplay devices 136 and input devices 138 may be used to display a userinterface that may include various graphical user interface (GUI)features that are usable to perform inspection operations, and/or tocreate and/or modify part programs, to view the images captured by thecamera system 260, and/or to directly control the vision componentsportion 200.

In various exemplary implementations, when a user utilizes the machinevision inspection system 100 to create a part program for the workpiece20, the user generates part program instructions by operating themachine vision inspection system 100 in a learn mode to provide adesired image-acquisition training sequence. For example, a trainingsequence may comprise positioning a particular workpiece feature of arepresentative workpiece in the field of view (FOV), setting lightlevels, focusing or autofocusing, acquiring an image, and providing aninspection training sequence applied to the image (e.g., using aninstance of one of the video tools on that workpiece feature). The learnmode operates such that the sequence(s) are captured or recorded andconverted to corresponding part program instructions. Theseinstructions, when the part program is executed, will cause the machinevision inspection system to reproduce the trained image acquisition andcause inspection operations to automatically inspect that particularworkpiece feature (that is the corresponding feature in thecorresponding location) on a run mode workpiece, or workpieces, whichmatches the representative workpiece used when creating the partprogram.

FIG. 3 is a schematic diagram of a VFL (TAG) lens system 300 (alsoreferred to as imaging system 300) that includes a TAG lens 370. The TAGlens system 300 may be adapted to a machine vision system or configuredas a standalone system, and may be operated according to principlesdisclosed herein. In certain applications, the VFL (TAG) lens system 300may be utilized as part of an “inline” measurement system (e.g., forwhich workpieces 320 may be moving at a high rate of speed relative tothe components of the system, and for which the principles disclosedherein may provide particular advantages, such as in relation to higherillumination energy being provided in shorter image exposure timeframes,as will be described in more detail below). It will be appreciated thatcertain numbered components 3XX of FIG. 3 may correspond to and/orprovide similar operations or functions as similarly numbered components2XX of FIG. 2, and may be similarly understood unless otherwiseindicated.

As will be described in more detail below, an imaging optical path OPATH(also called a workpiece imaging optical path herein) comprises variousoptical components arranged along a path that conveys image light 355from the workpiece 320 to the camera 360. The image light is generallyconveyed along the direction of their optical axes OA. In theimplementation shown in FIG. 3, all the optical axes OA are aligned.However, it will be appreciated that this implementation is intended tobe exemplary only and not limiting. More generally, the imaging opticalpath OPATH may include mirrors and/or other optical elements, and maytake any form that is operational for imaging the workpiece 320 using acamera (e.g., the camera 360) according to known principles. In theillustrated implementation, the imaging optical path OPATH includes theTAG lens 370 (which may be included in a 4f imaging configuration) andis utilized at least in part for imaging a surface of a workpiece 320during a workpiece image exposure.

As shown in FIG. 3, the VFL lens system 300 includes a light source 330,an exposure (strobe) time controller 333 es, an objective lens 350, atube lens 351, a relay lens 352, the VFL (TAG) lens 370, a relay lens356, a lens controller 380, a camera 360, an effective focus position(Z-height vs. phase) calibration portion 373 ph, an effective focusposition (Z-height vs. amplitude) calibration portion 373 am, and aworkpiece focus signal processing portion 375 (optional). In variousimplementations, the various components may be interconnected by directconnections or one or more data/control busses (e.g., a system signaland control bus 395) and/or application programming interfaces, etc.

As will be described in more detail below, in various implementations,the VFL lens controller 380 may control a drive signal of the TAG lens370 to periodically modulate optical power of the TAG lens 370 over arange of optical powers that occur at respective phase timings withinthe periodic modulation. The camera 360 (e.g., including an imagingdetector) receives light transmitted along an imaging optical path OPATHthrough the TAG lens 370 during an image exposure and provides acorresponding camera image. The objective lens 350 inputs image lightarising from a workpiece 320 during an image exposure, and transmits theimage light along the imaging optical path OPATH through the TAG lens370 to the camera 360 during the image exposure, to provide a workpieceimage in a corresponding camera image. An effective focus position EFPin front of the objective lens 350 during an image exposure correspondsto the optical power of the TAG lens 370 during that image exposure. Theexposure time controller 333 es is configured to control an imageexposure timing used for a camera image.

With respect to the general configuration shown in FIG. 3, the lightsource 330 may be a “coaxial” or other light source configured to emitthe source light 332 (e.g., with strobed or continuous illumination)along a path including a beam splitter 390 (e.g., a partially reflectingmirror as part of a beam splitter) and through the objective lens 350 toa surface of a workpiece 320. The objective lens 350 receives the imagelight 355 (e.g., workpiece light) that is focused at an effective focusposition EFP proximate to the workpiece 320, and outputs the image light355 to the tube lens 351. The tube lens 351 receives the image light 355and outputs it to the relay lens 352. In other implementations,analogous light sources may illuminate the field of view in anon-coaxial manner; for example, a ring light source may illuminate thefield of view.

In various implementations, the objective lens 350 may be aninterchangeable objective lens, and the tube lens 351 may be included aspart of a turret lens assembly (e.g., similar to the interchangeableobjective lens 250 and the turret lens assembly 223 of FIG. 2). In theimplementation shown in FIG. 3, image light 355 arising from a nominalfocal plane of the objective lens 350 is focused by the tube lens 351 toform an intermediate image at a nominal intermediate image plane IIPnom.When the TAG lens 370 is in a state where it provides no lensing effect(no optical power), the nominal focal plane of the objective lens 350,the nominal intermediate image plane IIPnom, and the image plane of thecamera 360 form a set of conjugate planes, according to known microscopeimaging principles. In various implementations, any of the other lensesreferenced herein may be formed from or operate in conjunction withindividual lenses, compound lenses, etc.

The relay lens 352 receives the image light 355 from the tube lens 351(or more generally from an intermediate image plane, in variousalternative microscope configurations) and outputs it to the TAG lens370. The TAG lens 370 receives the image light 355 and outputs it to therelay lens 356. The relay lens 356 receives the image light 355 andoutputs it to the camera 360. In various implementations, the camera 360captures a camera image during an image exposure (e.g., during anintegration period of the camera 360) also referred to as an imageexposure period, and may provide the corresponding image data to acontrol system portion. Some camera images may include a workpiece image(e.g., of a region of the workpiece 320) provided during a workpieceimage exposure. In some implementations, an image exposure (e.g., aworkpiece image exposure) may be limited or controlled by a strobetiming of the light source 330 that falls within an image integrationperiod of the camera 360. In various implementations, the camera 360 mayhave a pixel array greater than 1 megapixel (e.g., 1.3 megapixel, with a1280×1024 pixel array, with 5.3 microns per pixel).

In the example of FIG. 3, the relay lenses 352 and 356 and the VFL (TAG)lens 370 are designated as being included in a 4f optical configuration,while the relay lens 352 and the tube lens 351 are designated as beingincluded in a Keplerian telescope configuration, and the tube lens 351and the objective lens 350 are designated as being included in amicroscope configuration. All of the illustrated configurations will beunderstood to be exemplary only, and not limiting with respect to thepresent disclosure. In various implementations, the illustrated 4foptical configuration permits placing the VFL (TAG) lens 370 (e.g.,which may be a low numerical aperture (NA) device) at the Fourier planeof the objective lens 350. This configuration may maintain thetelecentricity at the workpiece 320, and may minimize scale change andimage distortion (e.g., including providing constant magnification foreach effective focus position (Z-height) of the workpiece 320). TheKeplerian telescope configuration (e.g., including the tube lens 351 andthe relay lens 352) may be included between the microscope configurationand the 4f optical configuration, and may be configured to provide adesired size of the projection of the objective lens clear aperture atthe location of the VFL (TAG) lens 370, so as to minimize imageaberrations, etc.

In various implementations, the lens controller 380 may include a drivesignal generator portion 381, a timing clock 381′, and imagingcircuits/routines 382. The drive signal generator portion 381 mayoperate (e.g., in conjunction with the timing clock 381′) to provide aperiodic drive signal to the high speed VFL (TAG) lens 370 via a signalline 380′ (e.g., for providing and/or adjusting an amplitude drivingsignal, as will be described in more detail below). In variousimplementations, the VFL lens system (or imaging system) 300 maycomprise a control system (e.g., the control system portion 120 of FIG.2) that is configurable to operate in conjunction with the lenscontroller 380 for coordinated operations.

In various implementations, the lens controller 380 may generallyperform various functions related to imaging a workpiece 320 in a mannersynchronized with a desired phase timing (or a desired peak focusdistance timing as associated with an amplitude driving signal) of theTAG lens 370, as well as controlling, monitoring and adjusting thedriving and response of the TAG lens 370. In various implementations,the image circuits/routines 382 perform standard imaging operations forthe optical system, synchronized with the phase timings (or the peakfocus distance timings as associated with amplitude driving signals) ofthe TAG lens 370.

In various instances, drift in the operating characteristics of the VFLlens may arise due to unwanted temperature variations. As shown in FIG.3, in various implementations, the imaging system 300 may optionallyinclude the lens heater/cooler 337 associated with the TAG lens 370. Thelens heater/cooler 337 may be configured to input an amount of heatenergy into the TAG lens 370 and/or perform cooling functions tofacilitate heating and/or cooling of the TAG lens 370 according to someimplementations and/or operating conditions. In addition, in variousimplementations, a TAG lens monitoring signal may be provided by atemperature sensor 336 associated with the TAG lens 370 to monitor anoperating temperature of the TAG lens 370.

With respect to the general operations of the TAG lens 370, in variousimplementations as described above, the lens controller 380 may rapidlyadjust or modulate its optical power periodically, to achieve ahigh-speed VFL lens that periodically modulates its optical power at aTAG lens resonant frequency of 400 kHz, 250 kHz, 70 kHz, or 30 kHz,etc., i.e., at a high speed. As shown in FIG. 3, by using the periodicmodulation of a signal to drive the TAG lens 370, the effective focusposition EFP of the imaging system 300 (that is, the focus position infront of the objective lens 350) may be rapidly moved within a rangeRefp (e.g., a focus range or an autofocus search range, etc.) bound byan effective focus position EFP1 (or EFPmax or peak focus distanceZ1max+) corresponding to a maximum optical power of the TAG lens 370 incombination with the objective lens 350, and an effective focus positionEFP2 (or EFPmin or peak focus distance Z1 max−) corresponding to amaximum negative optical power of the TAG lens 370 in combination withthe objective lens 350. In various implementations, the effective focuspositions EFP1 and EFP2 may approximately correspond to phase timings of90 degrees and 270 degrees, as will be described in more detail below.For purposes of discussion, the middle of the range Refp may bedesignated as EFPnom, and may approximately correspond to zero opticalpower of the TAG lens 370 in combination with the nominal optical powerof the objective lens 350. According to this description, EFPnom mayapproximately correspond to the nominal focal length of the objectivelens 350 in some implementations (e.g., which may correspond to aworking distance WD of the objective lens 350).

In some implementations, the optional focus signal processing portion375 may input data from the camera 360 and may provide data or signalsthat are utilized to determine when an imaged surface region (e.g., of aworkpiece 320) is at an effective focus position. For example, a groupof images acquired by the camera 360 at different effective focuspositions (Z-heights), such as part of an image stack, may be analyzedusing a known “maximum contrast” or “best focus image” analysis todetermine when an imaged surface region of a workpiece 320 is at acorresponding effective focus position (Z-height). However, moregenerally, any other suitable known image focus detection configurationmay be used. In any case, the workpiece focus signal processing portion375 or the like may input an image or images acquired during theperiodic modulation of the effective focus position (during the sweepingof multiple effective focus positions) of the TAG lens 370, anddetermine an image and/or image timing at which a target feature (e.g.,of a workpiece) is best focused.

In some implementations, the focus signal processing portion 375 maydetermine a phase timing (or an amplitude driving signal with anassociated peak focus distance timing) corresponding to a best focus(e.g., of a workpiece feature) and output that “best focus” phase timingvalue (or the “best focus” amplitude driving signal having the “bestfocus” peak focus distance timing) to an effective focus positioncalibration portion 373 ph (or 373 am).

The effective focus position (Z-height vs. phase) calibration portion373 ph may store “phase” calibration data determined by calibrationprocesses such as those disclosed in the incorporated references. Theeffective focus position calibration portion 373 ph may provideeffective focus position (Z-height vs. phase) calibration data thatrelates respective effective focus positions (Z-heights) to respective“best focus” phase timings within a period of a resonant frequency ofthe TAG lens 370.

The effective focus position (Z-height vs. amplitude) calibrationportion 373 am may store “amplitude” calibration data determined bycalibration processes such as those described in more detail below. Theeffective focus position calibration portion 373 am may provideeffective focus position (Z-height vs. amplitude driving signal)calibration data that relates respective effective focus positions(Z-heights) to respective “best focus” amplitude driving signals fordriving a resonant frequency of the TAG lens 370.

Generally speaking, the effective focus position calibration portions373 ph and 373 am comprise recorded effective focus position (Z-height)calibration data. As such, the representations in FIG. 3 of thecalibration portions 373 ph and 373 am as separate elements are intendedto be a schematic representation only, and not limiting. In variousimplementations, the associated recorded effective focus position(Z-height) calibration data 373 ph and 373 am may be merged with and/orindistinguishable from the lens controller 380, the workpiece focussignal processing portion 375, or a host computer system connected tothe system signal and control bus 395, etc.

In various implementations, the exposure (strobe) time controller 333 escontrols an image exposure time of the imaging system 300 (e.g.,relative to a phase timing of the periodically modulated effective focusposition). More specifically, in the standard (phase) mode, during animage exposure, the exposure (strobe) time controller 333 es may use theeffective focus position (Z-height) calibration data available in theeffective focus position (Z-height vs. phase) calibration portion 373 phand control the light source 330 to strobe at a respective time. In theamplitude adjustment mode, during an image exposure, the exposure(strobe) time controller 333 es may provide an exposure increment timeinterval that is approximately centered at the timing of either peakfocus distance Z1max+ or Z1 max− in reference to the calibration dataavailable in the effective focus position (Z-height vs. amplitude)calibration portion 373 am and control the light source 330 to strobe ata respective controlled time.

For example, the exposure (strobe) time controller 333 es may controlthe strobe light source to strobe at a respective phase timing within aperiod of a standard imaging resonant frequency of the TAG lens 370, soas to acquire an image having a particular effective focus positionwithin the sweeping (periodic modulation) range of the TAG lens 370. Inother implementations, the exposure time controller 333 es may control afast electronic camera shutter of the camera 360 to acquire an image ata respective controlled time and/or its associated effective focusposition. In some implementations, the exposure (strobe) time controller333 es may be merged with or indistinguishable from the camera 360. Itwill be appreciated that the operations of the exposure time controller333 es and other features and elements outlined above may be implementedto govern workpiece image acquisitions.

Still referring to FIG. 3, the VFL lens system (or imaging system) 300includes a standard mode (phase mode) portion 340 ph and an amplitudeadjustment mode portion 340 am, which respectively correspond to thestandard mode (phase mode) portion 140 ph and the amplitude adjustmentmode portion 140 am of FIG. 2. The standard mode (phase mode) portion340 ph is responsible for operating the TAG lens 370 in a standard (or“normal”) mode, in which a single amplitude driving signal (e.g., amaximum amplitude driving signal) is utilized to drive the periodicmodulation of the TAG lens 370 optical power, and calibration data fromthe effective focus position (Z-height vs. phase) calibration portion373 ph is utilized to determine phase timings of the periodic modulationthat correspond to Z-heights. The amplitude adjustment mode portion 340am is responsible for operating the TAG lens 370 in an amplitudeadjustment mode, in which amplitude driving signals having desired (bestfocus) amplitude peak timings are used to drive the periodic modulationof the TAG lens 370 optical power, in reference to the effective focusposition (Z-height vs. amplitude) calibration portion 373 am. In theamplitude adjustment mode, exposure increment time intervals are onlyprovided as corresponding to selected peak phase timings (e.g., 90° and270°), as will be more fully discussed below. In various embodiments, auser can make a selection for switching between the standard (phase)mode and the amplitude adjustment mode. Alternatively or additionally,the imaging system 300 may make a selection for switching between themodes in response to certain conditions or functions.

FIG. 4A is a timing diagram 400A illustrating phase timings for aperiodically modulated control signal 410 and optical response 420 ofthe VFL lens system of FIG. 3. In the example of FIG. 4A, an ideal caseis illustrated in which the control signal 410 and the optical response420 have similar phase timings and are thus represented as identicalsignals, although for which it will be understood that in some instancesthe signals may be separated by a phase offset, as described in U.S.Pat. No. 9,736,355, which is commonly assigned and is herebyincorporated by reference herein in its entirety. In variousimplementations, the control signal 410 may be related to the drivesignal (e.g., including an amplitude driving signal) that is produced bythe drive signal generator 381 of FIG. 3, and the optical response 420may be representative of the periodically modulated focus position ofthe imaging system which is controlled by periodically modulating theoptical power of the TAG lens 370, as outlined above.

In various implementations, the sinusoidal shapes of the curves 410 and420 may depend on the lenses in series (e.g., the objective lens 350,TAG lens 370, etc. as illustrated in FIG. 2), for which the opticalpower of the TAG lens 370 goes through a cycle as indicated in FIG. 4Aand is equal to 1/f (where f=focal length). As will be described in moredetail below, a Z-height versus phase calibration that relatesrespective Z-heights to respective phase timing signal values may beestablished by calibration according to known principles (e.g., inaccordance with a mathematical model and/or by repeatedly stepping asurface to a known Z-height, and then manually or computationallydetermining the phase timing that best focuses an image at the knownZ-height, and storing that relationship in a lookup table or the like inthe effective focus position (Z-height vs. phase) calibration portion373 ph).

The timing diagram 400A illustrates phase timings (e.g., ϕ0, ϕ90, ϕ180,ϕ270, etc.) which are equal to respective phase timing signal values(e.g., t0, t90, t180, t270, etc.) of the control signal 410, whichcorrespond to respective Z-heights (e.g., zϕ0, zϕ90, zϕ180, zϕ270, etc.)In various implementations, the phase timing signal values (e.g., t0,t90, t180, t270, etc.) may be determined according to a phase timingsignal (e.g., as provided by a clock or other technique for establishinga timing relative to the periodic modulation, etc.) It will beunderstood that the phase timing signal values shown in the timingdiagram 400A are intended to be exemplary only and not limiting. Moregenerally, any phase timing signal value will have an associated focusposition Z-height within the illustrated range of focus positions (e.g.,the range in the illustrated example having a maximum Z-height zϕ90 anda minimum Z-height zϕ270).

As described above, various techniques (e.g., utilizing points fromfocus, maximum confocal brightness determinations, etc.) may be used todetermine when an imaged surface region is in focus, which maycorrespond to a Z-height measurement for the imaged surface region. Forexample, an imaged surface region may be determined to be at a Z-heightzϕsurf when the imaged surface region is in focus. In the illustratedexample utilizing the phase vs Z-height principles, at the phase timingϕsurf_ind(−), which is equal to the phase timing signal valueTsurf_ind(−) the focus position is at the Z-height zϕsurf, and aworkpiece surface region located at the Z-height zϕsurf is in focus.Similarly, at the phase timing ϕsurf_ind(+), which is equal to the phasetiming signal value Tsurf_ind(+), the focus position is at the Z-heightzϕsurf, and the workpiece surface region located at the Z-height zϕsurfis in focus. It will be appreciated that such values may be included inthe effective focus position (Z-height vs. phase) calibration portion373 ph that relates respective Z-heights to respective phase timingsignal values, such that when an imaged surface region is determined tobe in focus, the corresponding phase timing signal value (e.g.,Tsurf_ind(−)) may be utilized to look-up the corresponding measuredZ-height (e.g., Z-height zϕsurf) of the imaged surface region.

In the illustrated example, the phase timing signal values Tsurf_ind(−)and Tsurf_ind(+) correspond to movements of the modulated focus positionin respective opposite directions. More specifically, the phase timingsignal value Tsurf_ind(−) corresponds to movement of the modulated focusposition in a first direction (e.g., downward), while the phase timingsignal value Tsurf_ind(+) corresponds to movement of the modulated focusposition in a second direction (e.g., upward) that is opposite to thefirst direction.

FIG. 4A also qualitatively shows how strobed illumination can be timedto correspond with a respective phase timing (e.g., ϕ0, ϕ90, ϕ180, ϕ270,etc.) of the periodically modulated focus position to expose an imagefocused at a respective Z height (e.g., zϕ0, zϕ90, zϕ180, zϕ270, etc.).That is, in the illustrated example, while a digital camera is acquiringan image during an integration period, if a short strobe pulse isprovided at the phase timing ϕ0, then the focus position will be at theheight zϕ0, and any workpiece surface that is located at the height zϕ0will be in focus in the resulting image. The same will be true for theother exemplary phase timings and Z heights shown in the diagram 400A.

FIG. 4B shows certain details of one exemplary implementation of acontrolled timing CT that may be used to define an effective focusposition FP and certain other characteristics to determine acorresponding image exposure increment EI. In particular, the controltiming CT may be implemented in a light source strobe operation orcamera shutter strobe operation (e.g., as controlled by an exposure timecontroller 333 es) to determine the effective focus position FP andcertain other characteristics of a corresponding image exposureincrement EI. Certain related techniques are described in more detail inU.S. Pat. No. 10,178,321, which is hereby incorporated herein byreference in its entirety.

In the implementation shown FIG. 4B, each controlled timing CTicomprises a respective increment time Ti and a respective exposureincrement time interval Di, and a respective increment illuminationintensity Li is used during the respective exposure increment timeinterval Di. In particular, the illustrated controlled timing CT1 thatdetermines the exposure increment EI₁ comprises an increment time T1 anda respective exposure increment time interval D1 (e.g., a timed strobeduration). The illustrated controlled timing CT2 that determines theexposure increment EI₂ similarly comprises a respective increment timeT2 and an exposure increment time interval D2. It may be seen that eachexposure increment time interval is located to provide a central oraverage increment time that corresponds to a desired focus position. Forexample, the exposure increment time interval D1 is located to providethe increment time T1(=tz1) corresponding to the desired effective focusposition FP₁(=Z1), and the exposure increment time interval D2 islocated to provide the increment time T2(=tz5) corresponding to thedesired effective focus position FP₂(=Z5). In various implementations, arespective increment illumination intensity Li is used during arespective exposure increment time interval Di, and each discrete imageexposure increment is exposed using a combination of its respectiveincrement illumination intensity Li and its respective exposureincrement time interval Di such that the product (Li*Di) isapproximately the same for each of the discrete image exposureincrements. Such techniques tend to provide equal “weighting” at each ofthe desired focus positions, which may be advantageous in someimplementations.

The implementation shown in FIG. 4B also illustrates that an exposureincrement (EI₂) corresponding to an effective focus position (FP2),which is relatively closer to the middle of the focus range Refp,comprises a combination of an exposure increment time interval D2 whichis relatively shorter and an increment illumination intensity (e.g., L2,not shown) which is relatively larger, and an exposure increment (EI₁)corresponding to an effective focus position (FP1), which is relativelyfarther from the middle of the focus range Refp, comprises a combinationof a second exposure increment time interval D1 which is relativelylonger and a second increment illumination intensity (e.g., L1, notshown) which is relatively smaller. In various implementations whereinthe periodically modulated focus position changes approximatelysinusoidally as a function of time, this allows the product (Li*Di) tobe approximately the same for each of the discrete image exposureincrements while at the same time allowing each respective exposureincrement time interval to be controlled to provide approximately thesame amount of focus position change ΔFP during each exposure incrementtime interval. For example, it may be seen that this allows ΔFP1=ΔFP2 inFIG. 4B, even though the rate of focus change is different for eachexposure increment due to the sinusoidal focus modulation. Suchtechniques tend to provide another aspect of equal “weighting” of imagesat each of the desired focus positions, which may be advantageous insome implementations.

While such techniques provide certain advantages, it will be appreciatedthat for certain applications the product (Li*Di) may be relativelylimited (e.g., as one example (L2*D2) may be relatively limited giventhe relatively short exposure increment time interval D2 which isutilized to achieve the desired amount of focus position change ΔFP2).More specifically, even if the second increment illumination intensityL2 corresponds to the maximum illumination that a light source canprovide, the relatively short exposure increment time interval D2 limitsthe total amount of illumination for the imaging process. In addition,in order to provide more illumination in such configurations, the systemmay be required to repeat the exposure time increment over multiplecycles of the modulation (e.g., the exposure increment time interval D2may be repeated at the corresponding phase timing during a number ofcycles of the modulation in order to provide a desired total amount ofillumination for the image exposure, for which each cycle adds to theoverall exposure timeframe).

For certain applications (e.g., dark workpieces, rapidly movingworkpieces, etc.) it may be desirable to provide more illumination forimages and to provide the illumination more quickly. For example, forcertain “inline” measurement applications, workpieces may be laterallymoving at a high speed relative to the VFL (TAG) lens system. As will bedescribed in more detail below, by utilizing different amplitude drivingsignals and relatively long exposure increment time intervals (e.g., asapproximately centered at a timing of a peak focus distance), arelatively large amount of illumination energy may be provided forimaging during a relatively small amount of focus change (e.g., andduring a fewer number of cycles of the modulation, such as during only asingle or a few cycles). Utilization of such principles as disclosedherein may result in brighter images by at least 10×, 20×, or 30× ascompared to the prior art (e.g., and which in some implementations maybe provided in at least 10× shorter image exposures, due in part to thelarge amount of illumination that can provided during a single cycle ofthe modulation rather than requiring additional cycles of themodulation, etc.)

FIG. 5 is a timing diagram illustrating a first amplitude of theperiodic modulation 511 of the VFL (TAG) lens system of FIG. 3 inresponse to a first amplitude driving signal 510 as operating in theamplitude adjustment mode, and also illustrating for comparison certainphase timings as would be utilized in a standard phase mode. Theamplitude driving signal 510 may be the drive signal that is produced bythe drive signal generator 381 of FIG. 3, and the periodic modulation511 may be representative of the periodically modulated focus positionof the VFL (TAG) lens system 300 which is controlled by the amplitudedriving signal 510. In the example of FIG. 5, an ideal case isillustrated (similar to the example of FIG. 4A) in which the amplitudedriving signal 510 and the periodic modulation 511 (e.g., the opticalresponse) have similar phase timings and are thus represented asidentical signals, although for which it will be understood that in someinstances the signals may be separated by a phase offset, as describedfor example in the previously incorporated '355 patent.

In the amplitude adjustment mode, the amplitude driving signal 510 isused to drive the periodic modulation 511 of the TAG lens optical powerat a resonant frequency of the TAG lens 370. In FIG. 5, the horizontalaxis represents time, and the vertical axis represents Z-heights of theperiodic modulation 511 and the amplitude of the periodically modulatedcontrol signal 510 (the amplitude driving signal 510) for the TAG lens370, wherein the amplitude is normalized to a maximum value of 1.0. Aswill be described in more detail below, a Z-height versus amplitudecalibration that relates respective Z-heights to respective amplitudedriving signals with associated peak focus distance timings may beestablished by calibration (e.g., in accordance with a mathematicalmodel and/or by repeatedly stepping a surface to a known Z-height, andthen manually or computationally determining the amplitude drivingsignal with a peak timing that best focuses an image at the knownZ-height, and storing that relationship in a lookup table or the like inthe effective focus position (Z-height vs. amplitude) calibrationportion 373 am).

In FIG. 5, the amplitude driving signal 510 is set to vary over amaximum range from 1.0 to −1.0. This results in a first amplitude of theperiodic modulation 511, which corresponds to a first focal Z rangeextending between peak focus distances Z1 (i.e., Z1max+) and Z6 (i.e.,Z1max−). In various implementations, the first amplitude driving signal510 may be selected based on a first target focus distance Z6′. As willbe described in more detail below, for the periodic modulation 511, afirst peak focus distance timing T6 may be approximately centered withinan exposure increment (EI) time interval 551, and for which the targetfocus distance Z6′ may fall within (e.g., approximately centered in) anexposure Z range 553.

As will be described in more detail below with respect to FIGS. 6A-6F,different amplitude driving signals may be selected/utilized thatcorrespond to different focal Z ranges of the periodic modulation. Forexample, FIGS. 6A and 6B illustrate the same first amplitude drivingsignal as FIG. 5, which corresponds to a first focal Z range (i.e., fromZ1 to Z6), FIGS. 6C and 6D illustrate a second amplitude driving signalthat corresponds to a second focal Z range (i.e., from Z2 to Z5) andFIGS. 6E and 6F illustrate a third amplitude driving signal thatcorresponds to a third focal Z range (i.e., from Z3 to Z4). For purposesof comparison with the maximum amplitude driving signal 510, each ofthese ranges is illustrated in FIG. 5. The ranges and corresponding peakZ-heights also help illustrate certain aspects in relation to phasetimings as would be utilized in a standard phase mode, as will bedescribed in more detail below.

As illustrated in FIG. 5, if a standard phase mode was being utilized,the phase timings of T1, T2, and T3 (i.e., for positive optical powers)may be utilized to correspond to heights Z1, Z2, and Z3, respectively,and the phase timings of T6, T5 and T4 (i.e., for negative opticalpowers) may be utilized to correspond to Z-heights Z6, Z5 and Z4,respectively. As will be described in more detail below, in accordancewith the amplitude adjustment mode, rather than utilizing the differentphase timings to correspond to the different Z heights, differentamplitude driving signals may be utilized.

In relation to certain aspects as illustrated in FIG. 5, a differencebetween the values of 1.0 and 0.0 for positive optical powerscorresponds to +1 DPT, and a difference between values of 0.0 and −1.0for negative optical powers corresponds to −1 DPT. As used herein, DPTmeans optical power of the TAG lens 370 in diopter and represents onehalf of a resonant range (focal Z range) of the periodic modulation ofthe TAG lens optical power at a resonant frequency of the TAG lens 370.

In accordance with various exemplary embodiments, the imaging system 300may be operated in the amplitude adjustment mode, in which an amplitudedriving signal is used to drive a periodic modulation of the TAG lensoptical power (e.g., so that a target focus distance will fall within anexposure Z range of an image exposure increment time interval thatcorresponds to a peak (or trough) of the periodic modulation). In FIG.5, an exposure increment (EI) time interval 551 is approximatelycentered at the timing of a negative peak focus distance Z1 max− ascorresponding to the Z height Z6. Due to the sinusoidal focus modulationas illustrated in FIG. 5, at the peak timing, the rate of opticalresponse change (focus distance change, or focus position change, ΔFP)is relatively small, which makes it possible to make the correspondingexposure increment time interval (e.g., a timed strobe duration)relatively long (e.g., as approximately centered at the peak timing). Inother words, for a given maximum amount of ΔFP (focus position change),which defines an exposure Z range 553, the longest correspondingexposure increment time interval is at a peak timing. As a result, itbecomes possible to provide a relatively large amount of illuminationenergy for imaging a workpiece (e.g., with a surface at a targetdistance within the exposure Z range 553), during the (relatively long)exposure increment time interval 551, as shown in FIG. 5. The capabilityto apply such higher illumination energy for imaging in turn results inbrighter images by at least 10×, 20×, or 30× as compared to the priorart.

In some implementation examples, when the exposure Z range 553 is 1 DOF,the corresponding exposure increment time interval 551 may be about2.032 μs for a 70 kHz resonant frequency, and when the exposure Z range553 is 2 DOF, the corresponding exposure increment time interval 551 maybe about 2.774 μs for the 70 kHz resonant frequency (e.g., as comparedto certain prior systems with a corresponding exposure increment timeinterval of about 0.080 μs for a 70 kHz resonant frequency).

While the above describes the higher illumination energy imaging at thenegative peak focus distance Z1 max− as corresponding to the Z heightZ6, the same higher illumination energy imaging is similarly possible ata positive peak focus distance Z1max+ as corresponding to the Z heightZ1, as will be apparent to those skilled in the art. In the illustratedexample, the peak timings correspond to phase timings of 90° and 270°,though the corresponding phase timings are not limited to this exampleand may be, for example 0° and 180° or any other phase timings (e.g., asseparated by 180°).

Thus, FIG. 5 illustrates how the depicted amplitude driving signal 510can be used to drive the periodic modulation of the TAG lens opticalpower, wherein the amplitude corresponds to a focal Z range (“Focal Zrange 1” in FIG. 5) extending between peak focus distance Z1max+ and Z1max−. As will be more fully described below, the Z-height vs. amplitudecalibration data in the effective focus position (Z-height vs.amplitude) calibration portion 373 am relates different amplitudedriving signals peak focus distance timing to different target focusdistances Ztarget−i. Thus, if a particular target focus distanceZtarget−1 is known, the calibration data may indicate which amplitudedriving signal corresponds to the particular target focus distanceZtarget−1 and thus is suitable for measuring and/or imaging a surfacelocated at the particular target focus distance Ztarget−1. In theexample of FIG. 5, the calibration data may indicate that the amplitudedriving signal 510, which can measure/image the Z height Z6′, issuitable for measuring/imaging a particular target focus distanceZtarget−1 if Ztarget−1 is known or expected to be at or near the Zheight Z6′ (or at or near a Z height Z1′ as will be described in moredetail below with respect to FIG. 6A).

As noted above, with respect to first target focus distance Z6′, a firstpeak focus distance timing T6 may be approximately centered within anexposure increment (EI) time interval 551, and for which the targetfocus distance Z6′ may fall within (e.g., be approximately centered in)an exposure Z range 553. It is noted that the target focus distance Z6′may be near to but different than the Z-height Z6. This occurs becauseit may be desirable in various implementations for the target focusdistance Z6′ to fall within a middle of the exposure Z range 553 (i.e.,rather than occurring near or at one end of the exposure Z range). Thus,the calibration data that is stored (e.g., as part of calibration data373 am) may relate the amplitude driving signal 510 with a peak focusdistance timing of T6 (e.g., corresponding to a timing of 7 us in thescale of FIG. 5) to the Z-height Z6′ rather than the Z-height Z6. Invarious implementations, the calibration data may also or alternativelystore the phase timing corresponding to the start of the exposureincrement time interval 551 and/or such timing may be determined byother components of the system in relation to the peak focus distancetiming of T6 (e.g., in order to determine the start time for anillumination pulse corresponding to the exposure increment time interval551). After the calibration data is stored, when a Ztarget−1 is known orexpected to be at or near the Z height Z6′, the calibration data willindicate that the amplitude driving signal 510 should be utilized (e.g.,with a exposure increment time interval 551 approximately centered atthe peak focus distance timing of T6).

Still referring to FIG. 5, if the target focus distance Ztarget−1 isinstead near the Z heights Z2 or Z5 defining “Focal Z range 2,” or nearthe Z heights Z3 or Z4 defining “Focal Z range 3,” then the depictedamplitude driving signal 510 is not suitable for measuring or imagingthe target focus distance Ztarget−1 because Ztarget−1 is not at or nearthe positive or negative focus peaks corresponding to the amplitudedriving signal 510. Thus, a different amplitude driving signal should beselected whose corresponding focus peaks approximately coincide with theknown or expected Ztarget−1 (e.g., near Z2 or Z5, or Z3 or Z4).

FIGS. 6A and 6B, 6C and 6D, and 6E and 6F respectively illustrate use ofthree different amplitude driving signals 510, 512, and 514, suitablefor measuring or imaging the target focus distance Ztarget−1 located ator near the Z heights of Z1′ or Z6′, Z2′ or Z5′, and Z3′ or Z4′,respectively (e.g., as may be near the Z heights of Z1 or Z6, Z2 or Z5,and Z3 or Z4, similar to the relationship of Z6′ to Z6, as describedabove).

FIGS. 6A and 6B correspond to FIG. 5 described above. FIGS. 6A and 6Billustrate that a first amplitude driving signal 510 is used to drive aperiodic modulation 511 of the TAG lens optical power at a resonantfrequency of the TAG lens 370. The first amplitude driving signal 510provides a first amplitude of the periodic modulation 511 at theresonant frequency, wherein the first amplitude corresponds to the firstfocal Z range, “Focal Z range 1,” extending between peak focus distancesZ1max+ (Z1) and Z1 max− (Z6). The first amplitude driving signal 510 isselected based on a first target focus distance Ztarget−1 and storedcalibration data, wherein the calibration data relates differentamplitude driving signals to different target focus distances Ztarget−iand indicates that the first amplitude driving signal 510 corresponds tothe first target focus distance Ztarget−1, which is expected to be at ornear the focus distances Z1′ or Z6′ (e.g., which in variousimplementations may be proximate to the peak focus distances Z1max+ (Z1)or Z1 max− (Z6), respectively).

In operation, the light source 330 and the camera 360 (FIG. 3) arecontrolled to expose an image using at least one exposure increment Ellhaving an exposure increment time interval EIT1, wherein the focusdistance during the exposure increment Ell moves over a first exposure Zrange EZR1 that includes the target focus distance Ztarget−1.

In various implementations, the exposure increment time interval EIT1 isapproximately centered at the timing of either peak focus distanceZ1max+ (Z1) or Z1 max− (Z6). In various implementations, the firstexposure Z range EZR1 is less than 2 DOF, such as 1 DOF, or is less than10% of the first focal Z range “Focal Z range 1.” In variousimplementations, Ztarget−1 falls within the first exposure Z range EZR1(e.g., falling approximately within the middle of the range). In variousimplementations, Ztarget−1 is proximate to either the peak focusdistance Z1max+ (Z1) or Z1 max− (Z6) in that the difference betweenZtarget−1 and the proximate peak focus distance is less than 10% of thefirst focal Z range “Focal Z range 1” or is less than 2 DOF.

In various implementations, the light source 330 is operated to providepulse light with a first phase pulse timing (e.g. 90° or 270°) thatcorresponds to the timing of the peak focus distance Z1max+ (Z1) or Z1max− (Z6). In various implementations, the camera 360 is operated toacquire an image of a first workpiece feature at the first target focusdistance Ztarget−1 as illuminated by the pulse light at the first phasepulse timing (e.g. 90° or 270°).

As used herein, that the first phase pulse timing “corresponds” to thetiming of the peak focus distance Z1max+ or Z1 max− means broadly that,in various implementations, the first phase pulse timing is configuredand timed so that the timing of the peak focus distance Z1max+ or Z1max−occurs during (e.g., approximately at the center of) the exposureincrement time interval. For example, the exposure increment timeinterval may in some implementations be equal to a pulse duration andmay begin at the first phase pulse timing. In some implementations, thephase pulse timing may correspond to the timing of the peak focusdistance Z1max+ or Z1 max− in that the first phase pulse timing mayprecede the timing of the peak focus distance Z1max+ or Z1max− by ½ thepulse duration (e.g., ½ the exposure increment time interval).

In various implementations, the exposure operation as illustrated inFIG. 6A or 6B may be cyclically repeated to provide repeated exposureincrements corresponding to the Z-height Z1′ or Z6′ to perform robustmeasurement/imaging. That is, while FIGS. 6A and 6B each depict onecycle of the first amplitude driving signal 510, two or more cycles ofthe first amplitude driving signal 510 may be used with correspondingexposure increments to expose an image corresponding to the Z-height Z1′or Z6′. In such implementations, one exposure of an image may beconsidered as comprising a plurality of exposure time increments thatare repeated cyclically, each using the same exposure increment timeinterval.

FIGS. 6C and 6D illustrate that a second amplitude driving signal 512 isused to drive a periodic modulation 513 of the TAG lens optical power ata resonant frequency of the TAG lens 370. The second amplitude drivingsignal 512 provides a second amplitude of the periodic modulation 513 atthe resonant frequency, wherein the second amplitude corresponds to thesecond focal Z range, “Focal Z range 2,” extending between peak focusdistances Z1max+ (Z2) and Z1 max− (Z5). The second amplitude drivingsignal 512 may be selected based on a second target focus distanceZtarget−2 and stored calibration data, wherein the calibration datarelates different amplitude driving signals to different target focusdistances Ztarget−i and indicates that the second amplitude drivingsignal 512 corresponds to the second target focus distance Ztarget−2,which is expected to be at or near the focus distances Z2′ or Z5′ (e.g.,which in various implementations may be proximate to the peak focusdistances Z1max+ (Z2) or Z1 max− (Z5), respectively).

In operation, the light source 330 and the camera 360 are controlled toexpose an image using at least one exposure increment EI₂ having anexposure increment time interval EIT2, wherein the focus distance duringthe exposure increment EI₂ moves over a second exposure Z range EZR2that includes the second target focus distance Ztarget−2.

In various implementations, the exposure increment time interval EIT2 isapproximately centered at the timing of either peak focus distanceZ1max+ (Z2) or Z1max− (Z5). In various implementations, the secondexposure Z range EZR2 is less than 2 DOF, such as 1 DOF, or is less than10% of the first focal Z range “Focal Z range 1.” (e.g., and in someimplementations may also be less than 10% of the second focal Z range“Focal Z range 2”). In various implementations, Ztarget−2 falls withinthe second exposure Z range EZR2 (e.g., falling approximately within themiddle of the range). In various implementations, the second targetfocus distance Ztarget−2 is proximate to the peak focus distance Z1max+(Z2) or Z1 max− (Z5) in that the difference between Ztarget−2 and theproximate peak focus distance is less than 10% of the first focal Zrange “Focal Z range 1” (e.g., and in some implementations may also beless than 10% of the second focal Z range “Focal Z range 2”) or is lessthan 2 DOF.

In various implementations, the light source 330 is operated to providepulse light with a phase pulse timing (e.g. 90° or 270°) thatcorresponds to the timing of the peak focus distance Z1max+ (Z2) or Z1max− (Z5). In various implementations, the camera 360 is operated toacquire an image of a workpiece feature at the second target focusdistance Ztarget−2 as illuminated by the pulse light at the phase pulsetiming (e.g. 90° or 270°).

In various implementations, the exposure operation as illustrated inFIG. 6C or 6D may be cyclically repeated to provide repeated exposureincrements corresponding to the Z-height Z2′ or Z5′ to perform robustmeasurement/imaging. That is, while FIGS. 6C and 6D each depict onecycle of the second amplitude driving signal 512, two or more cycles ofthe second amplitude driving signal 512 may be used with correspondingexposure increments to expose an image corresponding to the Z-height Z2or Z5. In such implementations, image exposure may be considered ascomprising a plurality of exposure time increments that are repeatedcyclically, each using the same exposure increment time interval.

FIGS. 6E and 6F illustrate that a third amplitude driving signal 514 isused to drive a periodic modulation 515 of the TAG lens optical power ata resonant frequency of the TAG lens 370. The third amplitude drivingsignal 514 provides a third amplitude of the periodic modulation 515 atthe resonant frequency, wherein the third amplitude 515 corresponds tothe third focal Z range, “Focal Z range 3,” extending between peak focusdistances Z1max+ (Z3) and Z1 max− (Z4). The third amplitude drivingsignal 514 may be selected based on a third target focus distanceZtarget−3 and stored calibration data, wherein the calibration datarelates different amplitude driving signals to different target focusdistances Ztarget−i and indicates that the third amplitude drivingsignal 514 corresponds to the third target focus distance Ztarget−3,which is expected to be at or near the focus distances Z3′ or Z4′ (e.g.,which in various implementations may be proximate to the peak focusdistances Z1max+ (Z3) or Z1max− (Z4), respectively).

In operation, the light source 330 and the camera 360 are controlled toexpose an image using at least one exposure increment EI3 having anexposure increment time interval EIT3, wherein the focus distance duringthe exposure increment EI3 moves over a third exposure Z range EZR3 thatincludes the third target focus distance Ztarget−3.

In various implementations, the exposure increment time interval EIT3 isapproximately centered at the timing of either peak focus distanceZ1max+ (Z3) or Z1max− (Z4). In various implementations, the thirdexposure Z range EZR3 is less than 2 DOF, such as 1 DOF, or is less than10% of the first focal Z range “Focal Z range 1.” In variousimplementations, Ztarget−3 falls within the first exposure Z range EZR3(e.g., falling approximately within the middle of the range). In variousimplementations, the third target focus distance Ztarget−3 is proximateto the peak focus distance Z1max+(Z3) or Z1 max− (Z4) in that thedifference between Ztarget−3 and the proximate peak focus distance isless than 10% of the first focal Z range “Focal Z range 1” or is lessthan 2 DOF.

In various implementations, the light source 330 is operated to providepulse light with a phase pulse timing (e.g. 90° or 270°) thatcorresponds to the timing of the peak focus distance Z1max+ (Z3) or Z1max− (Z4). In various implementations, the camera 360 is operated toacquire an image of a workpiece feature at the third target focusdistance Ztarget−3 as illuminated by the pulse light at the phase pulsetiming (e.g. 90° or 270°).

In regard to the implementations of FIGS. 6A-6F, it will be appreciatedthat if a constant exposure increment time interval EIT is used (e.g.,as corresponding to a constant pulse light interval or duration), all ofthe exposure increment time intervals EIT1-EIT3 will be the same, andthe corresponding exposure Z ranges EZR1-EZR3 will vary (e.g., asillustrated in FIGS. 6A-6F). More specifically, EZR2 may be smaller thanEZR1, and EZR3 may similarly be smaller than EZR2 (and may in someimplementations be approximately ½ the size of EZR1, for which the“Focal Z Range 3” is noted to be ½ the size of the “Focal Z Range 1”).In an implementation where the amplitude driving signal 510 and theperiodic modulation 511 correspond to a maximum amplitude driving signalof the system, as noted above the exposure increment time interval EIT1may be set so that the exposure Z range EZR1 is a certain value relativeto DOF and/or a percentage of the “Focal Z Range 1” (e.g., ascorresponding to an amount of focus change that is acceptable/specified,etc. for an exposure increment time interval). For example, the exposureincrement time interval EIT1 may be set so that the exposure Z rangeEZR1 is approximately 1 DOF (i.e., as may correspond to 1/25 of the“Focal Z Range 1” in some implementations), or is approximately 2 DOF(i.e., as may correspond to approximately 2/25 of the “Focal Z Range 1”in some implementations), or is approximately some other percentage ofthe “Focal Z Range 1” (e.g., 5%, 10%, etc.) In such implementations, ifa constant exposure increment time interval EIT is used (i.e., for whichall of the exposure increment time intervals EIT1-EIT3 will be thesame), as noted above the corresponding exposure Z ranges EZR2 and EZR3will be less than EZR1 (and thus would also correspondingly be a lowerpercentage of the “Focal Z Range 1”).

As described herein, in various implementations, different exposureincrement time intervals may be utilized. In some implementations, EIT2may be longer than EIT1, and EIT3 may be longer than EIT2. In one suchimplementation, the exposure increment time intervals may be implementedor otherwise set so as to result in the exposure Z ranges EZR being moreconsistent or even constant (e.g., wherein EZR1, EZR2 and EZR3 may besimilar or identical to one another). In various implementations, boththe exposure increment time intervals and the exposure Z ranges may bemade to vary (e.g., and in certain implementations, a light intensityfor the images may be made to vary as well). As will be described inmore detail below, in certain implementations, it may be desirable tohave a first exposure increment time interval utilized for theacquisition of a first set of images in an image stack (e.g., for oddnumbered images), and a second exposure increment time interval (i.e.,that is different than the first exposure increment time interval)utilized for the acquisition of a second set of images in an image stack(e.g., for even numbered images), such as may result in high dynamicrange imaging (e.g., permitting a single height map computation onfields of view including both very dark and very bright features ofinterest at the same time).

In various implementations, the exposure operation as illustrated inFIG. 6E or 6F may be cyclically repeated to provide repeated exposureincrements corresponding to the Z-height Z3′ or Z4′ to perform robustmeasurement/imaging. That is, while FIGS. 6E and 6F each depict onecycle of the third amplitude driving signal 514, two or more cycles ofthe third amplitude driving signal 514 may be used with correspondingexposure increments to expose an image corresponding to the Z-height Z3′or Z4′. In such implementations, one image exposure may be considered ascomprising a plurality of exposure time increments that are repeatedcyclically, each using the same exposure increment time interval.

FIG. 7 is a flow diagram showing one example of a method for operating aVFL lens system 300 comprising a TAG lens 370, to measure or image aworkpiece surface in a measurement/imaging volume of the TAG lenssystem, wherein the measurement/imaging volume may be on or adjacent tothe workpiece stage 210 (FIG. 2) on which the workpiece 20 having theworkpiece surface is placed.

Step 704 includes periodically modulating the optical power of the TAGlens 370 over a range of optical powers at an operating frequency of theTAG lens.

Step 706 includes driving a periodic modulation of the TAG lens systemoptical power, using a first amplitude driving signal that provides afirst amplitude of the periodic modulation at the resonant frequency.The first amplitude corresponds to a first focal Z range extendingbetween peak focus distances Z1max+ and Z1 max−. The first amplitudedriving signal is selected based on a first target focus distanceZtarget−1 and stored calibration data, wherein the calibration datarelates different amplitude driving signals to different target focusdistances Ztarget−i and indicates that the first amplitude drivingsignal corresponds to the first target focus distance Ztarget−1.

Step 708 includes operating a light source and a camera to expose animage using at least one exposure increment having an exposure incrementtime interval wherein the focus distance during the exposure incrementmoves over a first exposure Z range that includes the target focusdistance Ztarget−1.

In various implementations, different processes may be utilized fordetermining the phase calibration data 373 ph (e.g., which relatesZ-heights to phase timings) and the amplitude calibration data 373 am(e.g., which relates Z-heights to amplitude driving signals havingdifferent peak focus distance timings). For example, one exemplarytechnique for determining phase calibration data for a TAG lens isdescribed in co-pending U.S. application Ser. No. 16/232,874, which iscommonly assigned and is hereby incorporated herein by reference in itsentirety. As described in the '874 application, the techniques fordetermining the phase calibration data may involve a number of steps,and may involve certain modeling and may in some instances be relativelycomplex (e.g., in part due to the high rate of oscillation and for whichthe phase timings and corresponding determinations must be precise toaccurately determine the phase calibration data for the Z-heights).

In contrast, certain portions of the determination of the “amplitude”calibration data may be relatively less complex (e.g., in some instancespotentially resulting in certain higher levels of accuracy for certainportions of the amplitude calibration data and/or for which timing maybe a less critical factor).

In various implementations, for each amplitude driving signal of theamplitude calibration data, a Z-height that corresponds to the amplitudedriving signal may be determined utilizing a specified process (e.g.,such as the example process described below) for which the amplitudedriving signal may have corresponding level (e.g., a specified voltagelevel, etc.) that is set (e.g., thus simplifying certain portions of thedetermination process as compared to the determinations of the exactphase timings during the high rate of oscillation, etc.)

In various implementations, for calibration, imaging, and/or measurementprocesses, in a configuration utilizing a phase timing on a portion ofan oscillation curve with a high rate of change and with a shortexposure increment time interval and a short exposure Z range, if thephase timing for the exposure increment is off by a first minimal amount(e.g., such that the actual exposure increment time interval startsafter a time when the exposure increment time interval was supposed toend), the exposure Z range may miss and not include a desired targetfocus distance Ztarget. In contrast, in a system utilizing amplitudecalibration data as described herein, the rate of change on the utilizedportion of the oscillation curve may be much slower, and the exposuretime interval may be much longer. Thus, even if the phase timing for theexposure increment is off by the same first minimal amount, the exposureZ range is more likely to still include the desired target focusdistance Ztarget, and a majority of the exposure increment time intervalmay still provide the intended lighting (e.g., for imaging a workpiecesurface at the target focus distance Ztarget).

One exemplary laboratory calibration method to determine the amplitudecalibration data may employ a calibration surface (e.g., substitutingfor the surface of the workpiece 320 of FIG. 3) moved along the opticalaxis OA to different Z-heights (e.g., in steps of certain increments,such as approximately 0.1 or 0.2 measurement unit steps, for which invarious implementations the measurement units may be millimeters,microns, etc.). For each actual calibration Z-height surface position,an amplitude driving signal that results in approximately best focus ofthe system at the calibration surface may be determined. For example, animage stack may be acquired for a range of amplitude driving signals(e.g., as incremented in small steps over the range), and for which afocus curve may be determined based on focus metric values fm(k,i)calculated for captured images(i) exposed at different Z heightpositions Z(i) along the z-axis. A peak of the focus curve may indicatean amplitude driving signal that provides the best focus at thecalibration surface position at the Z-height. For each stepped positionof the calibration surface, the amplitude driving signal and thecorresponding actual Z-height position (in microns along the opticalaxis OA) are then recorded (e.g., and may be further processed accordingto fit curves etc.) to provide the amplitude calibration data that isstored (e.g., in a look-up table, etc.) in the effective focus position(Z-height vs. amplitude) calibration portion 373 am.

In various implementations, the stored amplitude calibration data may beutilized for various purposes. For example, in an implementation whereit is desired to focus the system at a target Z-height (e.g., where theworkpiece surface may be known to be or expected to be, or where animage is to be acquired, etc.), the calibration data may be utilized todetermine an amplitude driving signal to be utilized which correspondsto the target Z-height. In another example, during a workpiecemeasurement operation, a focus curve may be determined (e.g., byobtaining an image stack utilizing different amplitude driving signals)that indicates an amplitude driving signal that best focuses the systemat the surface of the workpiece. The best-focus amplitude driving signalmay then be referenced against the calibration data to determine thecorresponding Z-height of the surface of the workpiece.

In various implementations, for each amplitude driving signal, thecalibration data may include two corresponding Z-heights, ascorresponding to peak focus distances Z1max+ and Z1 max−, respectively.For example, the process described above may initially includedetermining calibration data as utilizing exposure increments having anexposure increment time interval that corresponds to peak focusdistances Z1max+ of multiple amplitude driving signals. In variousimplementations, the exposure increment time interval may beapproximately centered at the timing of the peak focus distance Z1max+for each amplitude driving signal. The process may then further includesimilarly determining calibration data as utilizing exposure incrementshaving an exposure increment time interval that corresponds to peakfocus distances Z1 max−. In various implementations, the exposureincrement time interval may be approximately centered at the timing ofthe peak focus distance Z1 max− for each amplitude driving signal.

In some application examples, a workpiece may have first and secondsurfaces (or a first workpiece may have a first surface and a secondworkpiece may have a second surface) that are at a first Z-height and asecond Z-height, respectively. In various implementations, the first andsecond surfaces may be stationary or may be moving relative to the fieldof view during the imaging/measurement process. In variousimplementations, first and second “target” Z-heights may be established(e.g., as corresponding to the first and second Z-heights of theworkpiece surfaces). The system may be configured, and a first amplitudedriving signal may be selected (e.g., in accordance with the calibrationdata) for imaging both surfaces. For example, the first amplitudedriving signal with the peak focus distance Z1max+ may correspond to thefirst target Z-height1 and the first amplitude driving signal with thepeak focus distance Z1 max− may correspond to the second targetZ-height2. For such a process, in various implementations the system mayinitially be adjusted (e.g., by utilizing the motor 294 of FIG. 2 toadjust the overall position of the optical assembly portion 205, etc.)so that the middle of the range Refp (e.g., designated as EFPnom) may beapproximately in the middle between the first and second target Zheights. The amplitude driving signal may then be selected and/orutilized for imaging the first and second surfaces (e.g., utilizingexposure increment time intervals that are approximately centered at thetiming of the peak focus distances Z1max+ and Z1 max−, respectively, asdescribed herein).

As described above in reference to the possible use examples of thecalibration data, during a workpiece measurement operation, a focuscurve may be determined by obtaining an image stack utilizing differentamplitude driving signals, wherein for each amplitude driving signal twoimages may be obtained, as corresponding to peak focus distances Z1max+and Z1 max−, respectively. The focus curve may then be utilized todetermine an amplitude driving signal with the corresponding peak focusdistance (e.g., Z1max+ or Z1 max−) that best focuses the system at thesurface of the workpiece. The determined best-focus amplitude drivingsignal in combination with the corresponding peak focus distance (e.g.,Z1max+ or Z1 max−) that produced the best focus may then be looked up inthe calibration data to determine the corresponding Z-height of thesurface of the workpiece.

FIG. 8 is a flow diagram showing one example of a method for operating aVFL lens system 300 comprising a TAG lens 370, with a workpiece surfacein a measurement/imaging volume of the VFL lens system 300, wherein themeasurement/imaging volume may be on or adjacent to the workpiece stage210 (FIG. 2) on which the workpiece 20 having the workpiece surface isplaced.

Step 804 includes periodically modulating the optical power of the TAGlens 370 over a range of optical powers at a resonant frequency of theTAG lens.

In Step 806, during a first time frame, two sub-steps 806A and 806B areperformed. Step 806A includes driving a periodic modulation of the TAGlens optical power, using a first amplitude driving signal that providesa first amplitude of the periodic modulation at the resonant frequency.The first amplitude corresponds to a first focal Z range extendingbetween peak focus distances Z1max+ and Z1 max−. Still during the firsttime frame, step 806B includes operating the light source 330 and camera360 to expose a first image using a first exposure increment having afirst exposure increment time interval that is approximately centered atthe timing of either the peak focus distance Z1max+ or Z1 max−.

In step 808, during a second time frame, two sub-steps 808A and 808B areperformed. Step 808A includes driving a period modulation of the TAGlens optical power, using a second amplitude driving signal thatprovides a second amplitude of the periodic modulation at the resonantfrequency that is different than the first amplitude of the periodicmodulation. The second amplitude corresponds to a second focal Z rangeextending between peak focus distances Z2max+ and Z2max−, wherein thesecond focal Z range is different than the first focal Z range. Forexample, this setting corresponds to FIGS. 6C and 6D described aboveincluding the second amplitude driving signal 512 and correspondingmodulation 513 that sweeps through the second focal Z range “Focal Zrange 2” extending between peak focus distance Z2max+ (Z2) andZ2max−(Z5), while the setting during the first time frame (step 806)described above corresponds to FIGS. 6A and 6B including the firstamplitude driving signal 510 and corresponding modulation 511 thatsweeps through the first focal Z range “Focal Z range 1” extendingbetween peak focus distance Z1max+ (Z1) and Z1max− (Z6).

Still during the second time frame, step 808B includes operating thelight source 330 and camera 360 to expose a second image using a secondexposure increment having a second exposure increment time interval thatis approximately centered at the timing of either the peak focusdistance Z2max+ or Z2max−.

Step 810 includes performing processing that includes determining focusmetric values for the first and second images. In variousimplementations, the processing includes comparing the focus metricvalues for the first and second images. In various implementations, theprocessing includes determining that a focus metric value for the secondimage is higher than a focus metric value for the first image. Forexample, the focus metric values may involve a calculation of thecontrast or sharpness of a region of interest in each of the first andsecond images, and the focus metric values may be compared to determinewhich of the first and second images is in better focus.

In various implementations of the method of FIG. 8, the TAG lens system300 may be operated to acquire a stack of images, which are exposed atrespective discrete focus positions FP using different amplitude drivingsignals corresponding to different Z heights (corresponding to thediscrete focus positions FP), and determine a Z height of a workpiecesurface by utilizing the stack of images. For example, a focus positionmay move through a range of Z height positions Z(i) along the z-axis(the focusing axis) and the TAG lens system 300 (including the camera360) may use different amplitude driving signals (i) to captureimages(i) at different positions Z(i) as part of an image stack. Foreach captured image(i), a focus metric fm(k,i) may be calculated basedon a region or sub-region of interest ROI(k) (e.g. a set of pixels) inthe image. The focus metric fm(k,i) are related to the correspondingposition Z(i) of the focus at the time that the image is captured. Thisresults in focus curve data (e.g. the focus metrics at the positionsZ(i)), which may be referred to simply as a “focus curve” or “autofocuscurve”. In one embodiment, the focus metric values may involve acalculation of the contrast or sharpness of the region of interest inthe image. The Z-height corresponding to the peak of the focus curve,which corresponds to the best focus position along the Z axis, is the Zheight for the region of interest used to determine the focus curve.

When a focus metric value is based on contrast as noted above, one wayto determine the contrast is to evaluate each pixel of the image, andcompare its color/brightness with the neighboring pixels. The image withthe highest overall contrast may be determined to be the image in thebest focus, and a Z focus position corresponding to the best focus imagemay be determined as the Z height of the surface of the workpiece 320.

In various implementations of the method of FIG. 8, the focus distanceduring the first exposure increment (step 806) moves over a firstexposure Z range that is less than 10% of the first focal Z range. Invarious implementations, the focus distance during the first exposureincrement (step 806) moves over a first exposure Z range that is lessthan 2 DOF of the VFL lens system. In various implementations, the focusdistance during the second exposure increment (step 808) moves over asecond exposure Z range that is less than 10% of the first focal Zrange. In various implementations, the focus distance during the secondexposure increment (step 808) moves over a second exposure Z range thatis less than 2 DOF of the VFL lens system.

In various implementations, the first image and the second image may bepart of an image stack obtained by the system operating in a points fromfocus (PFF) mode, and the focus metric values are processed to determinea Z-height of the first workpiece surface. Briefly, in the PFF mode, theTAG lens system 300 is operated to expose a stack of images (an imagestack) using an exposure sequence involving different amplitude drivingsignals. The PFF image exposure sequence defines a plurality of discreteimage exposure increments acquired at respective discrete focuspositions FP corresponding to respective amplitude driving signals usedto drive the periodic modulation of the TAG lens optical power. Theplurality of discrete image exposure increments may be each determinedby a respective instance of a light source strobe operation that has arespective controlled timing defined in the PFF image exposure sequence.The image stack is processed to determine or output a Z heightcoordinate map (e.g. a point cloud) that quantitatively indicates a setof 3 dimensional surface coordinates corresponding to a surface shape ofthe workpiece 20.

It may be desirable/advantageous in some implementations (e.g., forcertain implementations in which PFF is utilized) to have the firstexposure increment time interval (in step 806) be equal to the secondexposure increment time interval (in step 808), while for certain otherimplementations (e.g., related to general autofocus, etc.) it may beacceptable or desirable for the first exposure increment time intervalto differ from the second exposure increment time interval (e.g., forwhich the exposure increment time intervals may vary for the imagesthroughout an image stack). In some implementations, it may be desirableto have a first exposure increment time interval utilized for theacquisition of a first set of images in an image stack, and a secondexposure increment time interval (i.e., that is different than the firstexposure increment time interval) utilized for the acquisition of asecond set of images in an image stack. For example, in oneimplementation alternating exposure/light pulse durations (i.e.,exposure increment time intervals) may be utilized for acquiringalternating images in an image stack (e.g., with a first exposureincrement time interval utilized for odd numbered images in the imagestack and a second exposure increment time interval utilized for evennumbered images in the image stack). Such an implementation may in someapplications provide high dynamic range imaging. High dynamic rangeimaging would permit a single height map computation on fields of viewincluding both very dark and very bright features of interest at thesame time without having to acquire two separate image stacks withdifferent desired exposures for such features (e.g., providing moreexposure for dark features and less exposure for bright features, etc.)

In various implementations, the method of FIG. 8 may further include,during a third time frame, adjusting the TAG lens controller to drivethe periodic modulation of the TAG lens optical power at the resonantfrequency of the TAG lens, using a third amplitude driving signal thatprovides a third amplitude of the periodic modulation at the resonantfrequency that is different than the first amplitude and the secondamplitude of the periodic modulation. The third amplitude corresponds toa third focal Z range extending between peak focus distances Z3max+ andZ3max−, wherein the third focal Z range is different than the firstfocal Z range and the second focal Z range. Still during the third timeframe, the method includes operating the light source and camera toexpose a third image using a third exposure increment having a thirdexposure increment time interval that is approximately centered at thetiming of either the peak focus distance Z3max+ or Z3max−. For example,this setting during the third time frame may correspond to FIGS. 6E and6F described above including the third amplitude driving signal 514 andcorresponding modulation 515 that sweeps through the third focal Z range“Focal Z range 3” extending between peak focus distance Z3max+ (Z3) andZ3max−(Z4).

While preferred implementations of the present disclosure have beenillustrated and described, numerous variations in the illustrated anddescribed arrangements of features and sequences of operations will beapparent to one skilled in the art based on this disclosure. Variousalternative forms may be used to implement the principles disclosedherein.

All of the U.S. patents and U.S. patent applications referred to in thisspecification are incorporated herein by reference, in their entirety.Aspects of the implementations can be modified, if necessary to employconcepts of the various patents and applications to provide yet furtherimplementations. These and other changes can be made to theimplementations in light of the above-detailed description. In general,in the following claims, the terms used should not be construed to limitthe claims to the specific implementations disclosed in thespecification and the claims, but should be construed to include allpossible implementations along with the full scope of equivalents towhich such claims are entitled.

The invention claimed is:
 1. A variable focal length (VFL) lens system,comprising: a tunable acoustic gradient (TAG) lens; a TAG lenscontroller that controls the TAG lens to periodically modulate theoptical power of the TAG lens over a range of optical powers at anoperating frequency; a light source; an objective lens that inputsworkpiece light arising from a first workpiece surface, which isilluminated by the light source, and transmits the workpiece light alongan imaging optical path that passes through the TAG lens; a camera thatreceives the workpiece light transmitted by the TAG lens along theimaging optical path and provides a corresponding workpiece imageexposure; a memory for storing programmed instructions; and one or moreprocessors to execute the programmed instructions to perform operationsincluding: during a first timeframe: operating the TAG lens controllerto drive a periodic modulation of the TAG lens optical power at aresonant frequency of the TAG lens, using a first amplitude drivingsignal that provides a first amplitude of the periodic modulation at theresonant frequency, the first amplitude corresponding to a first focal Zrange extending between peak focus distances Z1max+ and Z1 max−; andoperating the light source and camera to expose a first image using afirst exposure increment having a first exposure increment time intervalthat is approximately centered at the timing of either the peak focusdistance Z1max+ or Z1 max−; during a second timeframe: adjusting the TAGlens controller to drive the periodic modulation of the TAG lens opticalpower at the resonant frequency of the TAG lens, using a secondamplitude driving signal that provides a second amplitude of theperiodic modulation at the resonant frequency that is different than thefirst amplitude of the periodic modulation, the second amplitudecorresponding to a second focal Z range extending between peak focusdistances Z2max+ and Z2max−, wherein the second focal Z range isdifferent than the first focal Z range; and operating the light sourceand camera to expose a second image using a second exposure incrementhaving a second exposure increment time interval that is approximatelycentered at the timing of either the peak focus distance Z2max+ orZ2max−; and performing processing that includes determining focus metricvalues for the first and second images.
 2. The system of claim 1,wherein the processing includes comparing the focus metric values forthe first and second images.
 3. The system of claim 1, wherein theprocessing includes determining that a focus metric value for the secondimage is higher than a focus metric value for the first image.
 4. Thesystem of claim 1, wherein the focus distance during the first exposureincrement moves over a first exposure Z range that is less than 10% ofthe first focal Z range.
 5. The system of claim 1, wherein the focusdistance during the first exposure increment moves over a first exposureZ range that is less than 2 DOF (depth of field) of the system.
 6. Thesystem of claim 1, wherein the focus distance during the second exposureincrement moves over a second exposure Z range that is less than 10% ofthe first focal Z range.
 7. The system of claim 1, wherein the firstexposure increment time interval is equal to the second exposureincrement time interval.
 8. The system of claim 1, wherein the firstexposure increment time interval is different than the second exposureincrement time interval.
 9. The system of claim 1, wherein the firstimage and the second image are part of an image stack obtained by thesystem operating in a points from focus (PFF) mode, and the focus metricvalues are processed to determine a Z-height of the first workpiecesurface.
 10. The system of claim 1, wherein the one or more processorsare configured to execute the programmed instructions to performoperations further including: during a third timeframe: adjust the TAGlens controller to drive the periodic modulation of the TAG lens opticalpower at the resonant frequency of the TAG lens, using a third amplitudedriving signal that provides a third amplitude of the periodicmodulation at the resonant frequency that is different than the firstamplitude and the second amplitude of the periodic modulation, the thirdamplitude corresponding to a third focal Z range extending between peakfocus distances Z3max+ and Z3max−, wherein the third focal Z range isdifferent than the first focal Z range and the second focal Z range; andoperate the light source and camera to expose a third image using athird exposure increment having a third exposure increment time intervalthat is approximately centered at the timing of either the peak focusdistance Z3max+ or Z3max−; and perform processing that includesdetermining focus metric values for the first, second and third images.11. A method for operating a variable focal length (VFL) lens systemcomprising a tunable acoustic gradient (TAG) lens and with a workpiecesurface in a measurement/imaging volume of the VFL lens system, themethod comprising: (a) periodically modulating the optical power of theTAG lens over a range of optical powers at a resonant frequency of theTAG lens; (b) during a first timeframe: driving a periodic modulation ofthe TAG lens optical power, using a first amplitude driving signal thatprovides a first amplitude of the periodic modulation at the resonantfrequency, the first amplitude corresponding to a first focal Z rangeextending between peak focus distances Z1max+ and Z1 max−; and operatinga light source and a camera to expose a first image using a firstexposure increment having a first exposure increment time interval thatis approximately centered at the timing of either the peak focusdistance Z1max+ or Z1 max−; (c) during a second timeframe: driving aperiodic modulation of the TAG lens optical power, using a secondamplitude driving signal that provides a second amplitude of theperiodic modulation at the resonant frequency that is different than thefirst amplitude of the periodic modulation, the second amplitudecorresponding to a second focal Z range extending between peak focusdistances Z2max+ and Z2max−, wherein the second focal Z range isdifferent than the first focal Z range; and operating the light sourceand camera to expose a second image using a second exposure incrementhaving a second exposure increment time interval that is approximatelycentered at the timing of either the peak focus distance Z2max+ orZ2max−; and (d) performing processing that includes determining focusmetric values for the first and second images.
 12. The method of claim11, wherein the processing includes comparing the focus metric valuesfor the first and second images.
 13. The method of claim 11, wherein theprocessing includes determining that a focus metric value for the secondimage is higher than a focus metric value for the first image.
 14. Themethod of claim 11, wherein the focus distance during the first exposureincrement moves over a first exposure Z range that is less than 10% ofthe first focal Z range.
 15. The method of claim 11, wherein the focusdistance during the first exposure increment moves over a first exposureZ range that is less than 2 DOF (depth of field) of the VFL lens system.16. The method of claim 11, wherein the focus distance during the secondexposure increment moves over a second exposure Z range that is lessthan 10% of the second focal Z range.
 17. The method of claim 11,wherein the first exposure increment time interval is equal to thesecond exposure increment time interval.
 18. The method of claim 11,wherein the first exposure increment time interval is different than thesecond exposure increment time interval.
 19. The method of claim 11,wherein the first image and the second image are part of an image stackobtained by the VFL lens system operating in a points from focus (PFF)mode, and the focus metric values are processed to determine a Z-heightof the first workpiece surface.
 20. The method of claim 11, furthercomprising: (e) during a third timeframe: driving a periodic modulationof the TAG lens optical power, using a third amplitude driving signalthat provides a third amplitude of the periodic modulation at theresonant frequency that is different than the first amplitude and thesecond amplitude of the periodic modulation, the third amplitudecorresponding to a third focal Z range extending between peak focusdistances Z3max+ and Z3max−, wherein the third focal Z range isdifferent than the first focal Z range and the second focal Z range; andoperating the light source and camera to expose a third image using athird exposure increment having a third exposure increment time intervalthat is approximately centered at the timing of either the peak focusdistance Z3max+ or Z3max−; wherein the processing performed in (d)includes determining focus metric values for the first, second, andthird images.